Organic electroluminescent devices using an organic substance are promising in applications as a cheap large-area full-color display device having a solid light-emitting solid-state device and a light source array for writing, and a number of developments have been carried out. In general, an organic electroluminescent device is constructed of a light emitting layer and a pair of counter electrodes disposed sandwiching the light emitting layer therebetween. When an electric field is applied between the both electrodes, an electron is injected from the cathode, and a hole is injected from the anode. The light emission is a phenomenon in which the electron and the hole are re-coupled in the light emitting layer, and energy is released as light during a time when the energy level is returned to a valence band from a conductor.
However, in the case of such an organic electroluminescent device, there is a serious problem that the luminous efficiency is very low as compared with inorganic LED devices and fluorescent tubes.
Almost all of organic electroluminescent devices which are currently proposed are ones utilizing fluorescent light emission obtained by a singlet exciton of an organic light emitting material. In a simple mechanism of the quantum chemistry, in the exciton state, a ratio of the singlet exciton from which fluorescent light emission is obtainable to the triplet exciton from which phosphorescent light emission is obtainable is 1/3. So far as the fluorescent light emission is utilized, only 25% of the exciton can be effectively applied so that the luminous efficiency is low.
On the other hand, if phosphorescence obtainable from the triplet exciton can be utilized, the luminous efficiency should be able to be enhanced. Actually, in recent years, organic electroluminescent devices utilizing phosphorescence with a phenylpyridine complex of iridium have been reported, and it is reported that such organic electroluminescent devices exhibit the luminous efficiency of 2 to 3 times as compared with the conventional organic electroluminescent devices utilizing fluorescence (for example, see U.S. Pat. No. 6,303,238, Applied Physics Letter, 1999, Vol. 75, page 4, and Japanese Journal of Applied Physics, 1999, Vol. 38, pages L1502 to L1504).
Most of phosphorescence organic electroluminescent devices have a device construction of anode/hole transport layer/light emitting layer/block layer/electron transport layer/cathode. Here, the hole transport layer is a layer for transporting a hole from the anode into the light emitting layer. The electron transport layer is a layer for transporting an electrode from the cathode into the light emitting layer. One of works of the block layer is to block diffusion of a triplet exciton formed in the light emitting layer, and the other work is to block a phenomenon that the hole passes through the light emitting layer into the electron transport layer and the cathode. In this way, by blocking the triplet exciton or the hole or the both by the block layer, it is possible to design to enhance the luminous efficiency.
However, according to this construction, it is impossible to block diffusion of the triplet exciton at the interface between the hole transport layer and the light emitting layer, and it is also impossible to block pass-through of the electron. As a result, there is still room for improvement in the luminous efficiency.
As a hole transport material in the hole transport layer, benzidine compounds which have a high hole mobility and into which a hole is liable to be injected from the anode are principally used. Above all, N,N′-dinaphthyl-N,N′-diphenylbenzidine (NPD) is frequently used. This NPD has a lowest energy level T1 of the triplet excited state of 2.3 eV, the value of which is smaller than T1 of usually used light emitting materials (i.e., from 2.5 to 2.6 eV). Therefore, the triplet exciton diffuses into the hole transport layer side, leading to a lowering of the luminous efficiency.
Also, NPD has an electron affinity as large as 2.4 eV so that an electron is liable to come thereinto from the light emitting layer and that an ability to block an electron is small. This is also a cause of lowering in the luminous efficiency. Further, if an electron is injected into NPD, an anion radical of NPD is generated. Since this anion radical is instable, the generation of an anion radical becomes a cause of deterioration in the durability.
For the purpose of improving the durability, there is proposed an attempt for using an aromatic hydrocarbon compound in the hole transport layer (for example, see JP-A-10-255985, JP-A-2000-182775 and JP-A-2000-182777). However, such an aromatic hydrocarbon compound is an anthracene compound, and the anthracene compound has a small energy difference between a highest occupied molecular orbital and a lowest unoccupied molecular orbital, and has a low ionization potential. For these reasons, this aromatic hydrocarbon compound functions as the hole transport material. Accordingly, this aromatic hydrocarbon compound cannot avoid diffusion of the exciton or pass-through of the electron and has low luminous efficiency. Also, this aromatic hydrocarbon compound has a small T1 and cannot be applied for organic electroluminescent devices utilizing phosphorescence.